In a cathode-ray tube (hereafter CRT) apparatus used in televisions and the like, electron beams are emitted from the electron gun and deflected by a magnetic field which is created by the deflection yoke provided on the periphery of the funnel of the CRT. These deflected electron beams scan over the panel, which results in visual display. Here, the panel provided with a screen, which is a face irradiated by the electron beams, does not have a spherical surface centering on the deflection center of the electron beams, and the distance between the deflection center and a point irradiated by the electron beams increases towards the perimeter of the screen. Consequentially, deviation of the electron beams becomes most significant in the four corners of the screen, which leads to one type of raster distortion, pincushion distortion, as shown in FIG. 9A.
As to pincushion distortion shown in FIG. 9A, the distortion in the x-direction, horizontal pincushion distortion, is usually corrected by a deflection circuit for horizontal pincushion distortion, whereas the distortion in the y-direction, vertical pincushion distortion, is eliminated or reduced by placing a pair of permanent magnets at the top and bottom front edges of the deflection yoke frame to the panel side (see, e.g. Japanese Patent Publication No. 58-20455 and No. 63-18836). With the aid of FIG. 9B, the following describes the principle of the distortion correction. FIG. 9B is a pattern diagram illustrating an influence on the electron beams above the tube axis of the CRT, which is exerted by the permanent magnet.
In reference to FIG. 9B, the permanent magnet is placed with the N pole on the right side in the x-direction, and the S pole, left, as shown in FIG. 9B. Each electron beam of R, G, and B travels in the direction of the tube-axis (i.e. in the direction out of the page). The permanent magnet creates a leftward magnetic field perpendicular to the tube-axis direction over the traveling range of electron beams. Due to the effect of this magnetic field, an upward Lorentz force acts upon the electron beams. Since the magnet is provided on the y-axis of the CRT apparatus, the electron beams scanning closer to the central part of the panel's screen in the horizontal direction, (i.e. the x-direction), experience a larger Lorentz force, which allows for correction of the pincushion distortion.
Although it is not shown in the figure, another permanent magnet is symmetrically placed at the bottom front edge of the deflection yoke, opposite to the one at the top deflection yoke in respect to the tube-axis, with the magnetic poles flipped. The pincushion distortion at the bottom of the screen is corrected by this permanent magnet located at the bottom.
When the CRT apparatus is activated, temperature of the apparatus starts increasing from the start of the activation. The temperature differential range is subjected to the ambient temperature of the environment in which the CRT apparatus is placed, but it can be, for instance, several tens of degrees Celsius (° C.). Thus, in the case that activating the apparatus results in an increase in the temperature thereof, the magnetization of the permanent magnet changes with a negative temperature characteristic. When the magnetization of the permanent magnet changes with a negative temperature characteristic, proper correction over the pincushion distortion cannot be maintained any longer.
As a countermeasure for this problem, a technique has been developed (see, Japanese Laid-Open Patent Application No. 2001-126642). In this, a magnetic substance made of a metal alloy having an attribute in which the permeability changes with a negative temperature characteristic is attached to the outer lateral face of the permanent magnet provided on the deflection yoke frame. This allows correction of the pincushion distortion to be maintained against temperature change of the apparatus.
As to a CRT apparatus, late years, there is a trend toward making the panel flat. However, such a CRT apparatus with a flat panel needs to be attached with a permanent magnet with a larger magnetization in order to correct pincushion distortion. For example, compared to a conventional CRT apparatus, a CRT apparatus with a panel like this requires the magnetization of the permanent magnet to be three to five times larger. Thus, in this type of CRT apparatus, change in the magnetization of the permanent magnets in response to temperature change becomes significant, and therefore, a problem has arisen where the method of distortion correction cited in Japanese Laid-Open Patent Application No. 2001-126642 above is not quite competent to correct the pincushion distortion against temperature change of the apparatus. In short, as to the permanent magnet with a large magnetization, the change in the magnetization against temperature change is substantial. And thus, even if the magnetic substance, which is made of a metal alloy having the attribute where the permeability changes with a negative temperature characteristic, is attached as above, sufficient adjustment cannot be made for change in the correction efficiency against the raster distortion in response to change in the magnetization of the permanent magnet.
An additional problem occurs since variation in the magnetization among individual permanent magnets increases when the permanent magnets have a larger magnetization. That is, proper correction of the pincushion distortion cannot be obtained when such a permanent magnet is used in the CRT apparatus. Such a problem, i.e. the variation in the magnetization of the permanent magnets, may be solved in theory; namely, by employing additional manufacturing steps that include screening over the permanent magnets and using only the most appropriate permanent magnets at the manufacturing stage of the CRT apparatus. However, adopting such a method is impractical cost wise.